Method of post-curing a benzoyl peroxide cured fluoroelastomer composition



3,452,126 METHOD OF POST-CURING A BENZOYL PEROXIDE CURED FLUOROELASTOMER COMPOSITION Jerry K. Sieron, Dayton, Ohio, assignor to the United States of America as represented by the Secretary of the Air Force No Drawing. Filed Dec. 9, 1966, Ser. No. 600,668 Int. Cl. B29b 25/00 U.S. Cl. 264-102 2 Claims ABSTRACT OF THE DISCLOSURE The invention described herein may be manufactured and used by or for the United States Government for governmental purposes without the payment to me of any royalty thereon.

This invention deals with low compression set materials for use as seals, gaskets, flexible connectors, and the like. More particularly, the invention deals with the preparation of novel low compression set, high temperature resistant, peroxide vulcanized fluoroelastomers for use as O-ring seals, gaskets, flexible connectors, and the like in the hydraulic and fuel systems of high Mach number aircraft.

The use of rubber and synthetic elastomers in seals and gaskets is well known. Rubber, for example, has found wide use in the sealing art because of its elasticity, it being important that a good seal material be not only compressible but that it resist compression and press back tightly against the surfaces being sealed.

Many elastomers, when first prepared and when under relatively normal temperatures and pressures, exhibit the properties necessary for a good seal but, while under compression and at elevated temperature, harden (take a set), and cease to press back. Such hardened seals cannot prevent leakage, particularly of fluids under pressure, and therefore have a limited usefulness.

Advances in modern technology have created demands for machines and devices which must operate in environments at relatively high temperatures. Many of these devices require the presence of fuel or lubricants under relatively high pressure. For example; jet propelled, high Mach number aircraft inculde devices which comprise high pressure seals exposed to temperatures at high as 500 F. for long periods of time. Rubber cannot be used as a seal in such devices and the Workers in the art have been investigating the potentialities of synthetic elastomers.

Among the synthetic elastomers vulcanized fluoroelastomers, because of their excellent chemical resistance and fair heat resistance, have been considered as potentially valuable sealing materials. Thus, diamine cured fluoroelastomers have been tested but have a high compression set. Specifically, they exhibit undue hardening and loss of resilience when operating at 500 F. Peroxide cured fluoro- United States Patent elastomers also have a high compression set and have not been employed as seals. Normal post curing operations involve step cures to 400 F. and, under these conditions, sponging of peroxide cured fluoroelastomers due to inadequate cross-linking is a severe problem.

OBJECTS It is therefore an object of this invention to provide a method for making a temperature resistant, low compression set elastomer for use as O-rings, gaskets, flexible connectors, and the like.

It is a specific object of the invention to treat a peroxide cured fluoroelastomer in a manner to effect the removal of volatile constituents therefrom and convert said fluoroelastomer into a novel and valuable sealing material.

I have now found that the foregoing and related objects can be attained in the method of post-curing a benzoyl peroxide cured fluoroelastomer composition to produce a temperature resistant, low compression set material; said method comprising the step of heating said composition under reduced pressure to effect the removal of volatile decomposition products.

In a preferred embodiment of the invention I heat the elastomer composition under a low pressure, substantially that of a vacuum; the temperature being elevated stepwise within the range of about 200-400 F. For example, I prefer to raise the temperature of the composition in about 50 F. increments and hold the composition at each temperature level for about one day. It will be apparent to one skilled in the art that there can be considerable variation in the temperatures chosen, the increments of temperature, the pressure, and the time of heating at each temperature; it being important only to remove a substantial proportion of the volatiles either present in the composition at the time of curing or developed subsequently as a result of the heating. Thus it appears that the successful removal of substantially all volatiles which develop up to a temperature of about 400 F. permits of the subsequent use of the elastomer composition as a seal at temperatures up to about 550 F.

In another preferred embodiment of the invention, I post cure a benzoyl peroxide cured fluoroelastomer composition at about 200 F. and at atmospheric pressure for about a day prior to carrying out the heating of the composition under reduced pressure. This step effects the the termination of free radical cross linking within the composition before the process of removing volatiles begins.

In yet another preferred embodiment of the invention and following the removal of substantially all volatiles, I post cure the composition by heating it in the range of about 400-550 F. This latter step can be carried out at atmospheric pressure.

The method is applicable to any fluoroelastomer; for example, polymers and copolymers of vinylidene fluoride, perfiuoropropylene, chlorotrifiuoroethylene, and the like.

EXAMPLE A vinylidene fluoride-perfluoropropylene copolymer parts by weight), magnesium oxide (15 parts), MT carbon black (30 parts), and benzoyl peroxide (3 parts) were mixed. The composition was formed into button-shaped gaskets which were vulcanized by press curing for 60 minutes at 260 F. using well-known procedures. The gaskets were then post cured 1n the sistant, low compression set material comprising the following sequence of steps: steps of (a) heating said composition at about 200 F.

Pressure, Tensile, Elongation, Hardness Time, hrs. Tomp.F. mm. Hg. Set, percent p.s.1. percent Shore A,pts.

24 200 24 200 24 250 24 300 24 350 24 400 72.8 s 500 56.7 16 500 41.8 96 500 760 26. 3 2, 285 ass 76 168 550 760 Substantially unchanged As indicated, the gaskets were tested for compression at about atmospheric pressure; (b) heating said comset. In this test the gaskets are held at 400 F. for 72 position in the range of about ZOO-400 F. under reduced hours under a 25 percent compression. The pressure is pressure to effect the removal of substantially all volatiles; then released and the gaskets are allowed to expand and (c) thereafter further post-curing the composition by freely. If the gasket button expands and returns to its heating it in the range of 400-550 F. at atmospheric original height, it has zero compression set. If it does pressure. not expand and retains its compressed height, it has 100 2. The method according to claim 1 wherein said repercent compression set. Test gaskets reached a comduced pressure is substantially that ofavacuum. pression set of 26.3% which, under the severe conditions of the test, is very, and surprisingly good. References Cited Control gaskets where were not heated in vacuum as UNITED STATES PATENTS indicated but which were post cured at atmospheric pres sure showed substantially 100 percent compression set and 3029473 4/1962 Greenberg 264-447 could not be heated to 500 F. Comparable diamine OTHER REFERENCES cured polymer buttons were hard and had substantially Levy & Vanderbilt, the Society of the Plastics Industry,

100 Percent compression Inc., Evaluation and Development of Glass Reinforced The tensile of 2285, the elongation of 385, and the Shore hardness of 76 in the tested gasket indicates that Petmleum Industry 1960 sectlon the latter still retains the general properties characteristic Conroy Honn Robb & Wolf Rubber Age Kel F of elastomers before they have been subjected to adverse Elastomer 1955 V 01 76 No 4 p 543450 (260/92 environments. 1 i a It is to be understood that the foregoing example and DONALD ARNOLD, primary Examiner. description are for the purposes of illustration only, and

that various changes may be made therein withount de- SILBAUGH: Assistant Examinera tin from the s i it and see e of th t'o P I cl aim: p r P e mven 1 n US. Cl. X.R.

1. The method of post-curing a benzoyl peroxide cured 264-236, 347; 26092.1 fluoroelastomer composition to produce a temperature re- 40 

